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Sizing Neutrals from TX to LV Panel

A bit of help/ advice needed if anyone has a moment to spare please.

I have carried out numerous cable calculations using Amtech (Trimble) over the years, but, have for the first time come accross an example where I cannot use the software.

Basically, I am looking at a project requiring 2 new TX's from UKPN, serving 2 new LV Switch Panels (Clients). UKPN have kindly given me all the necessary info on the ACB's etc. and also tell me that I can have 3 No. Al singles per phase up to 630mm CSA, but, I can only have 2 No. Al singles for the Neutral.

Now my helpful friend from Amtech support has just advised that you cannot select 3 cables per phase and only have 2 cables for neutral using the software, leaving me in a position where I have to (dare I say it) manually calc. the nuetral size.

What I do know is that my Amtech calc tells me that I need 3x 600mm CSA Al cables per phase, and it currently tells me I need 3x 600mm CSA neutrals (which I can not have).

I also know that the max permitted neutral size is 630mm CSA, which now leaves me in a postion of concern that being only permitted 2 neutrals, they may calculate out to be too large for the UKPN TX's.

Any help would be hugely appreciated by someone who should know better than to rely on Amtech all the time.

Thanks in advance

Adrian

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0 Former Community Member over 4 years ago

I think you've got yourself into that awkward position of being handed a requirement by a DNO (which for them on the network is perfectly normal to use reduced neutrals, as their documentation states the available KVA is substantially balanced) and then by design or default you are becoming an iDNO or a BNO.

You then usually have to comply with BS7671 and the rules for iDNO or BNO adoption, until you deliver power to each apartment and BS 7671 becomes the requirement.

In my world, we wouldn't tolerate a reduced neutral (in many cases we have 150 or 200% neutral conductors due to the harmonic rich nature of loads) - for domestic housing it won't usually be an issue (even with an ACB at 50% neutral)

Equally, you need to take a sensible assessment of the actual load - whilst the TX may be rated at 1250KVA, what is actually going through it - in air ratings should easily be less than 3 x 500mm2, even buried rating should be feasible for the same conductor size, if the diversity assessment is sensible. At 630mm2, in copper, it's pretty well bomb proof

Regards

OMS
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0 Former Community Member over 4 years ago

I think you've got yourself into that awkward position of being handed a requirement by a DNO (which for them on the network is perfectly normal to use reduced neutrals, as their documentation states the available KVA is substantially balanced) and then by design or default you are becoming an iDNO or a BNO.

You then usually have to comply with BS7671 and the rules for iDNO or BNO adoption, until you deliver power to each apartment and BS 7671 becomes the requirement.

In my world, we wouldn't tolerate a reduced neutral (in many cases we have 150 or 200% neutral conductors due to the harmonic rich nature of loads) - for domestic housing it won't usually be an issue (even with an ACB at 50% neutral)

Equally, you need to take a sensible assessment of the actual load - whilst the TX may be rated at 1250KVA, what is actually going through it - in air ratings should easily be less than 3 x 500mm2, even buried rating should be feasible for the same conductor size, if the diversity assessment is sensible. At 630mm2, in copper, it's pretty well bomb proof

Regards

OMS
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Sizing Neutrals from TX to LV Panel

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